A High Voltage Direct Current (HVDC) system employs a method for overcoming power loss occurring in long-distance power transmission. Such an HVDC system uses a method for converting AC power, produced in a power station, into DC power, transmitting the DC power, and converting the DC power back into AC power so as to reduce power loss. The HVDC system is provided with an HVDC controller to control the overall transmission system. The HVDC controller must continue to perform its functions even if externally supplied power is interrupted. The HVDC controller is provided with a power monitoring device so as to be stably and continuously supplied with power, and the power monitoring device related to this technology is disclosed in Japanese Patent Application Publication NO. 1995-281767.
Japanese Patent Application Publication NO. 1995-281767 relates to a power device for supplying electricity in response to a requested load, and is composed of duplicate power supply sources, a subtraction means, a first control means, a second control means, and a selection means.
The duplicate power supply sources are composed of a first power supply source and a second power supply source to supply electricity corresponding to a requested load, and the subtraction means detects the electricity supplied from the first power supply source and subtracts a predetermined amount from the electricity supplied from the first power supply source when the detected electricity is equal to or greater than a preset value. The first control means controls the electricity supplied from the second power supply source based on the difference resulting from the subtraction by the subtraction means while controlling the electricity supplied from the first power supply source in response to a requested load. The second control means detects the electricity supplied from the first power supply source, and controls the electricity supplied from the second power supply source in response to the requested load when the detected electricity is less than or equal to a preset value. The selection means compares the electricity supplied from the second control means with the amounts of electricity supplied from the first and second power supply sources, selects the larger amount of electricity, and supplies the selected electricity to the load, thus supplying duplicate power.
The conventional technology, such as Japanese Patent Application Publication NO. 1995-281767, is configured to duplicate power sources using one commercial power source and one emergency power source. When these power sources are applied to an HVDC controller, if the one emergency power source is interrupted while the one commercial power source is interrupted, the HVDC controller cannot continue to perform its function. That is, a problem arises in that, when driving power is interrupted, the HVDC controller is incapable of controlling the overall HVDC system and thus is incapable of continuing to control high DC voltage transmission, thus seriously deteriorating the reliability of power transmission using the HVDC system.